This invention is for use with composite trusses having wood chords and metal webbing as shown in Troutner, U.S. Pat. No. 3,137,899, June 23, 1964, Gilb, U.S. Pat. No. 3,857,218, Dec. 31, 1974 and Gilb's, Truss Structure with Clevis Assembly Joints, Ser. No. 758,061, filed Jan. 10, 1977 or the like.
In order for composite trusses to withstand the loads for which they are designed, it is essential that all elements of the truss remain in their original alignment. Any significant deviation in the alignment between the webs and chords is particularly critical because of the high moment forces which tend to overtune the chords. Rotation of the bottom chords will increase the moment forces and may result in premature failure of the truss and the structure carried by the truss. At each point, one web is in compression while the other is in tension. These forces are represented by arrows 2 and 3 which are opposite in direction. Since, as a practical matter, the webs are not attached to the truss pin at the exact same point, the arrows are separated by a moment arm 4. The moment of the two forces acting in opposite directions tend to cause the chord, in the instant case, to rotate in a clockwise direction as shown by arrow 6.
Prior to the present invention, truss designers did not recognize the problem of overturn and attempted to satisfy building code officials by providing single metal straps to connect the truss lower chord to the lateral bridging. As shown in FIG. 4, the prior art system (Trus Joist of Boise, Idaho) used a single metal strap 7 with one end 8 formed with an opening to receive pin 9 therethrough and the other end 11 was formed with fastener openings 12 for connection to the lateral bracing member 13. The metal strap was positioned between the metal webs 14 and 15 and thus was incapable of resisting the inherent overturning moments created by the compressions force 16 acting on web 15 and the tension force 17 acting on web 14. Thus the strap did not resist the clockwise turning force indicated by curved arrow 18 on lower chord member 19.
Sweets (U.S. patent application number unknown) appears to have recognized the problem of overturning moment forces and encircled the entire bottom and both sides of the wood chord; nailed the strap to the wood chord and then attached the free ends to the lateral bracing member. The Sweets strap uses far too much metal and more importantly, there is no attachment directly to the metal pin where the overturning force is occurring. With the Sweets strap, the overturning forces imposed by the webs act upon the pin and the forces are then transferred through the wood chord to the encircling Sweets strap. Further, the nails driven into the bottom wood chord in the Sweets method tend to split the wood chord and weaken it.